Starter/generator system with variable-frequency exciter control

ABSTRACT

A control for a brushless generator is operable in a starting mode of operation to convert electrical power into motive power for starting a prime mover. The control applies AC power to an exciter field winding at a first frequency at the initiation of a start sequence and thereafter decreases the frequency of the power applied to the exciter field winding during the start sequence.

TECHNICAL FIELD

The present invention relates generally to electromagnetic machines, andmore particularly to a control for a brushless generator operable instarting and generating modes.

BACKGROUND ART

An auxiliary power unit (APU) system is often provided on an aircraftand is operable to provide auxiliary and/or emergency power to one ormore aircraft loads. In conventional APU systems, a dedicated startermotor is operated during a starting sequence to bring a gas turbineengine up to self-sustaining speed, following which the engine isaccelerated to operating speed. Once this condition is reached, abrushless, synchronous generator is coupled to and driven by the gasturbine engine during operation in a starting mode whereupon thegenerator develops electrical power.

As is known, an electromagnetic machine may be operated as a motor toconvert electrical power into motive power. Thus, in those applicationswhere a source of motive power is required for engine starting, such asin an APU system, it is possible to dispense with the need for thededicated starter motor and operate the generator as a motor during thestarting sequence to accelerate the engine to self-sustaining speed.This capability is particularly advantageous in aircraft applicationswhere size and weight must be held to a minimum.

The use of a generator in starting and generating modes in an aircraftapplication has been realized in a variable-speed, constant-frequency(VSCF) power generating system. In such a system a brushless,three-phase synchronous generator operates in the generating mode toconvert variable-speed motive power supplied by a prime mover intovariable-frequency AC power. The variable-frequency power is rectifiedand provided over a DC link to a controllable static inverter. Theinverter is operated to produce constant-frequency AC power, which isthen supplied over a load bus to one or more loads.

The generator of such a VSCF system is operated as a motor in thestarting mode to convert electrical power supplied by an external ACpower source into motive power which is provided to the prime mover tobring it up to self-sustaining speed. In the case of a brushless,synchronous generator including a permanent magnet generator (PMG), anexciter portion and a main generator portion mounted on a common shaft,it has been known to provide power at a controlled voltage and frequencyto the armature windings of the main generator portion and to providefield current to the main generator portion field windings via theexciter portion so that the motive power may be developed. This has beenaccomplished in the past, for example, using two separate inverters, oneto provide power to the main generator portion armature windings and theother to provide power to the exciter portion. Thereafter, operation inthe generating mode may commence whereupon DC power is provided to theexciter field winding.

Patents disclosing systems in which a single dynamoelectric machine isutilized both as a motor to start an engine and as an AC generator whichproduces AC output power include Lafuze, U.S. Pat. No. 3,902,073,Messenger, U.S. Pat. No. 3,908,161, Hoffman, et al., U.S. Pat. No.4,093,869, Shilling, et al., U.S. Pat. No. 4,743,777, Dhyanchand, U.S.Pat. No. 4,939,441, Dhyanchand, et al., U.S. Pat. No. 4,947,100, Rozman,et al., U.S. Pat. No. 4,949,021, Dhyanchand, U.S. Pat. No. 4,968,926,Dhyanchand, U.S. Pat. No. 5,015,941, Dhyanchand, U.S. Pat. No. 5,055,700and Glennon, et al., U.S. Pat. No. 5,068,590.

Operation of the brushless generator as a motor and the subsequentconversion to generator operation can introduce a number ofdifficulties, particularly with respect to the excitation of themachine. In the generating mode, DC field current is supplied to themain generator portion field winding and a polyphase voltage isgenerated in the armature winding of the main generator portion.However, during operation in the starting mode, the machine is initiallyat a standstill so that a DC voltage applied to the exciter fieldwinding will not produce any AC voltage in the exciter armaturewindings. Thus, at standstill, it is necessary to convert the exciterinto a rotating transformer having a stationary primary winding excitedfrom an AC supply so that power can be supplied to the main generatorportion field winding. AC power can then be applied to the maingenerator portion armature winding to set up a rotating magnetic fieldwhich interacts with the magnetic field developed by the rotor so thatoperation in the starting mode can commence.

The above-identified Messenger '161 patent discloses a brushlessgenerator wherein a set of three exciter field windings are connected ina wye configuration during operation in the starting mode such that thestator windings and an exciter armature winding operate as a rotatingtransformer. After the generator has been brought to speed and is to beoperated in the generating mode, the stator field windings arereconnected into a series configuration.

The above-identified Shilling, et al. '777 patent discloses a brushless,synchronous generator having a three-phase AC exciter for use in thestarting mode and a DC exciter for use in the generating mode.

The above-identified Hoffman, et al. '869 patent discloses thecombination of a single-phase AC exciter for use in the starting modeand a DC exciter for use in the generating mode. The AC field winding isarranged in space-quadrature relation with respect to the DC fieldwinding. By this arrangement, the field which is created when the ACspace-quadrature winding is excited will not induce voltages into the DCfield winding, which is inactive during the starting mode.

The above-identified Glennon, et al. '590 patent discloses a three-phaseAC exciter which receives AC power during the motoring and generatingmodes.

Each of the foregoing patents discloses a system which requiresmodification of the existing generator and/or addition of a single-phaseor multiphase AC exciter to accomplish operation in the starting andgenerating modes.

An alternative method of operating a brushless generator in a startingmode has been proposed wherein a single-phase AC supply is connected tothe exciter field winding wherein the exciter operates as a rotatingtransformer. This approach, however, has been found to be notsatisfactory inasmuch as the energy transfer across the air gap in themachine is very small, and, therefore, the voltage induced in theexciter armature winding is very low. In order to overcome this problem,a very high AC voltage must be applied to the exciter field windingduring operation in the starting mode. This high voltage can causedamage to insulation and other components.

Okada, et al., U.S. Pat. No. 4,841,216 discloses the use of the sameexciter field coil for DC excitation during the generating mode and forAC excitation during the starting mode. The problems associated with theneed to apply high voltage is solved by switching at higher speedsduring the starting mode from high voltage AC excitation to DCexcitation. The high voltage AC excitation is obtained by utilizing atransformer as a means to step up AC voltage supplied by an AC powersource.

SUMMARY OF THE INVENTION

In accordance with the present invention, a control for a brushlessgenerator avoids the problems noted with respect to the prior art in asimple and inexpensive fashion.

More particularly, according to a first aspect of the present invention,a control for a brushless generator having a main generator portion anda permanent magnet generator (PMG) includes an exciter having an exciterfield winding disposed in a stator of the generator and an armaturewinding disposed on a rotor on the generator and coupled to a maingenerator portion field winding. First and second power converters areprovided wherein each power converter includes an input and an output.Means are operable in a starting mode of operation for coupling a sourceof electrical power to the inputs of the first and second powerconverters, the output of the first power converter to the maingenerator portion armature winding and the output of the second powerconverter to the exciter field winding. The coupling means is furtheroperable in a generating mode of operation for disconnecting the sourceof electrical power from the first and second power converters andconnecting a voltage regulator to the exciter field winding and theinput of one of the power converters to the main generator portionarmature winding. Means are coupled to the first and second powerconverters and operable in the starting mode for controlling the powerconverters such that the first and second power converters 10 providefirst and second AC waveforms at first and second frequencies to themain generator portion armature winding and to the exciter fieldwinding, respectively, at the beginning of the start sequence and suchthat the second AC waveform changes to a third frequency less than thesecond frequency after the beginning of the start sequence. Thecontrolling means are operable in the generating mode for controllingthe power converter coupled to the main generator portion armaturewinding such that AC power is produced at the output of such powerconverter.

Preferably, the controlling means comprises means for causing thefrequency of the second AC waveform to continuously decrease during thestart sequence until a particular rotor speed is reached. The causingmeans may also include means for applying DC power to the exciter fieldwinding after the particular rotor speed is reached. Preferably, theapplying means decreases a parameter of the DC power after a furtherparticular rotor speed is reached. In accordance with the preferredembodiment, the parameter of DC power comprises DC voltage magnitude.

Also preferably, the exciter field winding includes a mid-tap and firstand second end taps and the coupling means includes means for connectingthe mid-tap and one of the end taps to the second power converter duringoperation in the starting mode and for connecting the end taps to thevoltage regulator in the generating mode.

Still further in accordance with the preferred embodiment, the first andsecond power converters comprise first and second inverters,respectively, wherein an AC/DC converter is coupled between the sourceof electrical power and the first and second inverters during the startsequence.

The controlling means preferably comprises means for causing thefrequency of the first AC waveform to continuously increase during thestart sequence. The causing means preferably includes means fordetecting rotor position and means for commutating the main generatorportion armature winding based upon the detected rotor position.Further, the detecting means may comprise a sensorless rotor positiondetector.

In accordance with another aspect of the present invention, a controlfor a brushless generator having a main generator portion and apermanent magnet generator (PMG) includes an exciter having an exciterfield winding disposed in a stator and a set of armature windingsdisposed on a rotor and coupled to a field winding of the main generatorportion by a set of rotating rectifiers. First and second inverters anda rectifier bridge are provided each having an input and an output.Contactors are operable in the starting mode for coupling a source of ACpower to the input of the rectifier bridge, the output of the rectifierbridge to the inputs of the first and second power converters, theoutput of the first power converter to the set of main generator portionarmature windings and the output of the second power converter to theexciter field winding. The contactors are operable in the generatingmode for disconnecting the source of electrical power from the first andsecond inverters and connecting a voltage regulator to the exciter fieldwinding, the input of the rectifier bridge to the set of main generatorportion armature windings and the input of one of the power convertersto the output of the rectifier bridge. Means are coupled to the firstand second inverters and operable in the starting mode for controllingthe inverters such that the first and second inverters provide first andsecond AC waveforms at first and second frequencies to the set of maingenerator portion armature windings and to the exciter field winding,respectively, at the beginning of the start sequence and such that thefrequency of the second AC waveform continuously decreases during thestart sequence until a particular rotor speed is reached after thebeginning of the start sequence. The controlling means are operable inthe generating mode for controlling the power converter coupled to themain generator portion armature winding such that AC power is producedat the output of such power converter.

The control of the present invention includes a number of componentswhich are used both in the generating and starting modes. This highdegree of commonality leads to a desirable system simplification withattendant size and weight reduction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a combined schematic and block diagram of the control of thepresent invention in conjunction with a brushless generator;

FIG. 2 is a block diagram of the power conversion system of FIG. 1;

FIG. 3 is a combined schematic and block diagram of the exciter powerconverter of FIG. 2;

FIG. 4 is a combined schematic and block diagram of the exciter inverterand associated inverter control of FIG. 3;

FIG. 5 is a block diagram of the commutator, the main inverter and themain generator portion armature windings of FIG. 2;

FIG. 6 is a cross-section of a stator and a rotor of a synchronousmachine;

FIG. 7A is a diagram of the inverter, the main generator portionarmature phase windings, and a portion of the reluctance controller ofFIG. 5;

FIG. 7B is a diagram of a second portion of the reluctance controller ofFIG. 5;

FIG. 8 illustrates a number of signals generated during the operation ofthe power conversion system of FIG. 1;

FIG. 9 illustrates a number of signals generated 10 in the reluctancecontroller portion of FIG. 7B;

FIG. 10 illustrates a table specifying the output signals generated bythe logic unit of FIG. 7B;

FIG. 11 is a diagram of a portion of a first alternative embodiment ofthe reluctance controller of FIG. 5;

FIG. 12 is a diagram of a portion of a second alternative embodiment ofthe reluctance controller of FIG. 5;

FIG. 13 is a diagram of a portion of a third alternative embodiment ofthe reluctance controller of FIG. 5;

FIG. 14 is a block diagram of the back-EMF controller of FIG. 5;

FIG. 15 is a block diagram of the back EMF state observer of FIG. 14;

FIG. 16 is a block diagram of a first embodiment of the anglereconstruction unit of FIG. 14;

FIG. 16A is a block diagram of a first embodiment of thevoltage-to-angle converter of FIG. 16;

FIG. 16B is a block diagram of a second embodiment of thevoltage-to-angle converter of FIG. 16;

FIG. 17 is a block diagram of a second embodiment of the anglereconstruction unit of FIG. 14;

FIG. 18 is a block diagram of the speed controller of FIG. 14; and

FIG. 19 is a circuit diagram of the phase converter of FIG. 14.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, a brushless, synchronous generator 10 includesa permanent magnet generator (PMG) 12, an exciter portion 14 and a maingenerator portion 16. The generator 10 further includes a motive powershaft 18 connected to a rotor 20 of the generator 10. The motive powershaft 18 may be coupled to a prime mover 21, which may comprise, forexample, a gas turbine engine. The rotor 20 carries one or morepermanent magnets 22 which form poles for the PMG 12. Rotation of themotive power shaft 18 causes relative movement between the magnetic fluxproduced by the permanent magnet 22 and a set of three-phase PMGarmature windings including phase windings 24a-24c mounted within astator 26 of the generator 10.

The generator 10, the prime mover 21 and the power conversion system 54may together comprise an aircraft auxiliary power unit (APU) or maycomprise portions of any other power supply, as desired.

The exciter portion 14 includes a field winding 28 disposed in thestator 26 and a set of three-phase armature windings 30a-30c disposed onthe rotor 20. A set of rotating rectifiers 32 interconnect the exciterarmature windings 30a-30c and a main generator portion field winding 34also disposed on the rotor 20. A set of three-phase main generatorportion armature windings 36a-36c are disposed in the stator 26.

During operation in a generating mode, the PMG armature windings 24a-24care coupled through a rectifier 38 and voltage regulator 40 and a pairof switches 42, 44 to end taps 46a, 46b of the exciter field winding 28.As the motive power shaft 18 is rotated, power produced in the PMGarmature windings 24a-24c is rectified, regulated and delivered to thefield winding 28. AC power is produced in the armature windings 30a-30c,rectified by the rotating rectifiers 32 and applied to the maingenerator portion field winding 34. Rotation of the motive power shaft18 and the field winding 34 induces three-phase AC voltages in the maingenerator portion armature windings 36a-36c as is conventional.

Often, it is desirable to use the brushless generator 10 as a motor tobring the prime mover 21 up to self-sustaining speed. This operation isaccomplished by providing electrical power to the main generator portionfield winding 34 via the exciter 14, providing AC power to the maingenerator portion armature windings 36a-36c via lines 48a-48c andsuitably commutating the currents flowing in the windings 36a-36c tocause the motive power shaft 18 to rotate. In the present invention,this operation is achieved by connecting an external AC source 50 oranother source of electrical power via contactors 52a-52c to a powerconversion system 54. A series of switches 56a-56c, as well as theswitches 42, 44, are moved to the positions opposite that shown in FIG.1 so that the power conversion system 54 is connected to the set ofarmature windings 36a-36c and to the end tap 46b and a mid-tap 46c ofthe exciter field winding 28. The power conversion system 54 is operatedto supply power as appropriate to the windings 36a-36c and the winding28 to cause the motive power shaft 18 to rotate and thus develop motivepower.

During operation in the generating mode, the switches 56a-56c are placedin the positions shown in FIG. 1 and the power conversion system 54 isoperated to produce constant-frequency AC power on a load bus 60.

Referring now to FIG. 2, the power conversion system 54 includes anAC/DC converter 70 in the form of a rectifier bridge which receives ACpower from the power source 50 via contactors 52a-52c. The powerconversion system 54 also includes a first power converter comprising amain inverter 72 coupled between a DC bus or link having conductors 73a,73b, a fixed-frequency oscillator 80, and a mode switch 82. The powerconversion system 54 also includes a filter 74 and an auxiliary orexciter power converter 76.

During operation in the generating mode, the switches are in theposition shown in FIG. 2 and the contactors 52a-52c are opened. The maingenerator portion armature windings 36a-36c are thus coupled to theAC/DC converter 70, which in turn provides DC power to the main inverter72. This power may also be supplied to the exciter power converter 76;however, since the converter 76 is not operated in the generating mode,no power is supplied by the converter 76 to any of the components. Themain inverter 72 converts the DC power into constant-frequency AC powerwhich is filtered by the filter 74 and supplied to the load bus 60. Themain inverter 72 receives a frequency reference signal developed by afixed frequency oscillator 80 via the switch 82. The frequency referencesignal establishes the operating frequency of the main inverter 72.

During operation in the starting mode, the switches 42, 44, 56a-56c and82 are moved to the positions opposite those shown in FIG. 2. Inaddition, the contactors 52a-52c are closed. The AC/DC converter 70develops DC power which is supplied to the main inverter 72 and theexciter power converter 76.

During starting mode, the commutator 84 is coupled to the main inverter72 via the mode switch 82 and generates a set of inverter drive orcommand signals which are provided to the main inverter 72 via line 85to properly commute the currents flowing in the windings 36a-36c. Thecommutator 84 generates the command signals based on the phase voltagesat the main generator portion armature windings 36a-36c, which aredetected by the commutator 84 via lines 87a-87c, and the phase currentsin the windings 36a-36b, which are detected via a pair of currentsensors 88a-88b and provided to the commutator 84 via a pair of lines89a-89b.

At initiation of operation in the starting mode, first and second ACwaveforms at first and second frequencies are delivered by the first andsecond power converters to the main generator portion armature windings36a-36c and to the exciter field winding 28, respectively. The exciter14 acts as a rotary transformer having a primary winding comprising thefield winding 28 and secondary windings comprising the armature windings30a-30c so that AC power is induced in the armature windings 30a-30c.This AC power is rectified by the rotating rectifiers 32 and applied asDC power to the main generator portion field winding 34. Interaction ofthe resulting magnetic fields causes the rotor 20 to rotate relative tothe stator 26 so that the motive power shaft 18 is accelerated.

The frequency of the AC waveforms applied to the main generator portionarmature windings 36a-36c is continuously and preferably uniformlyincreased during the starting mode in a linear fashion.

Also, as described below, the frequency of the AC power applied to theexciter field winding 28 is continuously decreased in a linear fashionduring operation in the starting mode until a particular rotor speed isreached. Thereafter, DC power is applied at a variable magnitude duringthe remainder of the start sequence to avoid any modulation between theexcitation frequency of the exciter field winding 28 and that of themain generator portion armature windings 36a-36c.

Once a further particular rotor speed has been reached, which istypically the base speed of the generator, field weakening is necessaryowing to the back EMF developed by the main generator portion.Accordingly, the DC voltage magnitude applied to the exciter fieldwinding 28 is reduced at speeds equal to or greater than the furtherparticular rotor speed so that further rotor acceleration is possibleuntil the self-sustaining speed of the prime mover 21 is reached.

Following operation in the starting mode, operation may commence in thegenerating mode, as described above.

Exciter Power Converter 76

Referring now to FIG. 3, the exciter power converter 76 includes a DC/DCboost converter 100 which is in turn coupled to an exciter inverter 102.The boost converter 100 includes an inductor L1, a controllable switchQ1, a diode D1 and a capacitor C1. The controllable switch Q1 isoperated to cause the input voltage magnitude appearing on the DC busconductors 73a, 73b to be boosted to a level as needed to properlyenergize the exciter field winding 28. A function generator 106 isresponsive to the speed of the rotor 20 and provides a linearlydecreasing output with increasing rotor speed. A signal indicative ofrotor speed may be developed as described below in connection with thedescription of the commutator 84. A pulse-width modulator 108 develops apulse-width modulation (PWM) control signal comprising a series ofpulses having widths which are dependent upon the output of the functiongenerator 106. A gate drive circuit 110 develops a gate drive signal ofappropriate magnitude for the controllable switch Q1 from the output ofthe pulse-width modulator 108.

The exciter inverter 102 is operated by an inverter control 104, whichis responsive to a rotor position signal and a phase reference signal. Asignal indicative of rotor position may be developed as described belowin connection with the description of the commutator 84.

FIG. 4 illustrates a diagram of the inverter control 104 shownschematically in FIG. 3. Referring now to FIG. 4, a rotor speed signal ωis provided to a function generator 114 and a function generator 124.The rotor speed signal may be generated from a rotor position signal asdescribed in more detail below. The function generator 114 generates anoutput voltage which is a function of the rotor speed signal ω. Theoutput voltage of the function generator 114 decreases linearly withrotor speed up to a predetermined rotor speed, at which point it dropsto zero. The predetermined rotor speed may correspond to an electricalfrequency of approximately 100-400 Hz (the corresponding angular rotorspeed depends on the number of rotor poles). The voltage output from thefunction generator 114 is provided to a voltage-to-frequency (V/F)converter 116, which generates a frequency signal proportional to themagnitude of the voltage generated by the function generator 114. As aresult, the V/F converter 116 generates an AC signal having a frequencythat decreases linearly until the predetermined rotor speed, at whichpoint the AC signal becomes a DC signal.

In response to the rotor speed signal ω, the function generator 124generates a magnitude signal that remains constant until a predeterminedrotor speed, and thereafter decreases at speeds higher than thepredetermined rotor speed.

The outputs of the converter 116 and the function generator 124 areprovided to a multiplier 118, which generates a current reference signalhaving the frequency specified by the V/F converter 116 and themagnitude specified by the function generator 124.

The current reference signal generated by the multiplier 118 is providedto a noninverting input of a summer 130. A current feedback signalprovided to an inverting input of the summer 130 via a line 132 issubtracted from the current reference signal to generate a current errorsignal. The error signal is supplied to a PI regulator 134 which in turnproduces control signals which are processed by a gate drive circuit 136to derive gate drive signals. The gate drive signals control switchesQ2-Q5 which form a part of the exciter inverter 102. The exciterinverter further includes diodes D1-D4 wherein the switches Q2-Q5 andthe diodes D1-D4 are connected in a bridge configuration. A currentsensor 144 provides the current feedback signal to the summer 130described above. The switches Q4 and Q5, which are optional, areutilized to provide bidirectional current through the exciter fieldwinding 28.

As noted above, during operation in the starting mode, the switchesQ2-Q3 (and the switches Q4 and Q5, if used) are operated to provide ACand DC power to the exciter field winding 28 in accordance with themagnitude and frequency of the reference current signal provided by themultiplier 118.

During operation in the generating mode, the switches 42, 44 are movedto the positions shown in FIG. 1 and hence the exciter inverter 102receives the output of the voltage regulator 40.

Commutator 84

FIG. 5 illustrates a block diagram of the commutator 84 shown connectedto the main inverter 72 and the main generator portion armature windings36a-36c. The commutator 84 includes a back-EMF controller 160, a switch162 coupled to the back-EMF controller 160 via a line 164, and areluctance controller 166 coupled to the switch 162 via a line 168.

During starting mode, commutation or inverter drive signals are providedto the main inverter 72 via the lines 85a-85f based on either thedifferential reluctance between the windings 36a-36c or the back EMFproduced in the generator 10.

Initially during the starting mode, when the rotor 20 is at low speedand the magnitude of the back EMF generated in the windings 36a-36c isrelatively small, the commutation signals are generated by thereluctance controller 168, and the switch 162 occupies the positionshown in FIG. 5 to provide the commutation signals to the main inverter172.

When a rotor speed threshold is reached and the magnitude of the backEMF is sufficiently large, the switch 162 is switched to connect theline 164 to the main inverter 72 so that the commutation signalsgenerated by the back-EMF controller 160 are used to drive the inverter72.

The point at which the switch 162 is switched may be determined in anumber of ways, such as when the magnitude of the back EMF reaches apredetermined threshold and/or when the rotor 20 reaches a predeterminedrotational speed.

Reluctance Controller 166

The reluctance controller 166 and its theory of operation is describedin detail below in connection with FIGS. 6-13. Referring to FIG. 6, across section of a portion of a simplified one-pole version of thestator 26 and the rotor 20 are shown. The stator 26 has three phasewindings 36a-36c which comprise a central member 26a-26c about which thewire coils (not shown) are formed. Typically, the stator 26 would havemore poles.

The rotor 20 has a first end 20a and a second end 20b. As shown in FIG.6, the first rotor end 20a is aligned at an angle θ with respect to thephase winding 36a; the second rotor end 20b is aligned at an angle θ+60°with respect to the second phase winding 36b; and the second rotor end20b is aligned at an angle 60°-θ with respect to the third phase winding36c.

The reluctance, or magnetic path length, between various pairs of thethree phase windings 36a-36c varies as a function of rotor position inaccordance with the following equations:

    R.sub.ac =K.sub.1 +K.sub.2 cos θ cos(60°-θ),[1]

    R.sub.ab =K.sub.1 +K.sub.2 cos θ cos(θ+60°),[2]

where R_(ac) is the reluctance between the phase windings 36a, 36c,where R_(ab) is the reluctance between the phase windings 36a, 36b, K₁is a first constant, K₂ is a second constant, and θ is the angle definedin FIG. 6.

The differential reluctance between various pairs of the three phasewindings 36a-36c also varies as a function of rotor position. Thedifferential reluctance is the difference between the reluctance betweena first pair of windings and a second pair of windings. For example, 10the differential reluctance between phase windings 36b, 36c, referred toherein as R_(b-c), is the difference between R_(ab) -R_(ac). It shouldbe appreciated that the differential reluctance R_(b-c) is zero when therotor 20 is vertically aligned in FIG. 6. From equations [1] and [2]above, the differential reluctance R_(b-c) is as follows:

    R.sub.b-c =R.sub.ab -R.sub.ac                              [ 3]

    R.sub.b-c =K.sub.1 +K.sub.2 cos θ cos(θ+60°)-K.sub.1 -K.sub.2 cos θ cos(60°-θ)              [4]

    R.sub.b-c =K.sub.2 [cos θ cos(θ+60°)-cos θ cos(60°-θ)]                                  [5]

Equation [5] above can be further simplified to show that thedifferential reluctance R_(b-c) is proportional to sin 2θ. Thedifferential reluctances R_(a-c) and R_(a-b) can be shown to beproportional to sin 2(θ-60°) and sin 2(θ+60°), respectively.

FIG. 7A illustrates the main inverter 72 when it is connected to thephase windings 36a-36c by the switches 56a-56c (not shown in FIG. 7A)during the starting mode and a portion of the reluctance controller 166which generates a rotor position signal based on the differentialreluctance between successively selected pairs of phase windings36a-36c. The inverter 72 includes six controllable transistor powerswitches T₁ through T₆ and six flyback diodes connected in a bridgeconfiguration. The actuation of the power switches T₁ -T₆ is controlledby the inverter drive signals provided via the lines 85a-85f, whichsignals are shown as waveforms T1 through T6 in FIG. 8. The positiveportions of the waveforms T1-T6 may be pulse-width modulated (not shown)by a PWM carrier signal having a much higher frequency than that of thewaveforms T1-T6.

The reluctance controller 166 includes a phase selector 190 which alsoreceives the six drive signals T1-T6 on the lines 85a-85f and generatestherefrom three switch actuator signals on three lines 192a-192c whichare used to selectively activate three switches 194a-194c, each of whichhas an input connected to one of the phase windings 36a-36c. Each of theswitches 194a-194c has a first output, shown at the bottom left portionof each switch, which is connected to the noninverting input of asumming amplifier 200 via a line 202. Each of the switches 194a-194c hasa second output, shown at the bottom right portion of each switch, whichis connected to one of three noninverting inputs of a summing amplifier206 via one of three lines 208a-208c.

At any given time during the starting mode of operation, there iscurrent flowing through exactly two of the three phase windings 36a-36c,with the third phase winding having no current passing therethrough, orbeing "unenergized." The currents passing through the phase windings36a-36c are shown represented as I_(A), I_(B), I_(c), respectively, inFIG. 8.

The phase selector 190 generates the switch actuator signals on thelines 192a-192c so that the voltage generated on the unenergized phasewinding, which is generated as a result of transformer coupling to theenergized phase windings, is provided to the noninverting input of thesumming amplifier 200. That is accomplished by causing the switch 194connected to the unenergized phase winding to connect its input to theoutput shown at the bottom left portion of the switch.

If the phase winding to which a switch 194 is connected is energized,the switch input is connected to the output shown at the bottom rightportion of the switch, so that the voltages on the two energized phasewindings are provided to two of the noninverting inputs of the summingamplifier 206.

The switch positions as shown in FIG. 7A occur when the phase winding36b is unenergized and the phase windings 36a, 36c are energized. Theswitch actuator signals generated by the phase selector 190 on the lines192a-192c are designated S1-S3, respectively, in FIG. 8 and are shownwith respect to the waveforms T1-T6 from which they are generated. Theswitch actuator signal S1 has a high value when neither waveform T1 norT4 has a high value; the signal S2 has a high value when neitherwaveform T3 nor T6 has a high value; and the signal S3 has a high valuewhen neither waveform T2 nor T5 has a high value.

In operation during the starting mode, two of the three windings 36a-36care energized, leaving the third winding unenergized. The switches194a-194c are repeatedly switched, as described above, so that thevoltage on the unenergized winding is always provided to thenoninverting input of the summing amplifier 200 via the line 202 and thevoltages on the energized phase windings are always provided to thesumming amplifier 206. The amplifier 206 sums the voltages of the twoenergized windings, and the sum is provided to a divider 210 whichdivides the sum by the number of energized phase voltages used togenerate the voltage sum, which in this case is two, to generate anaverage phase voltage signal.

The voltage on the unenergized phase winding will have a relativelylarge DC component and a relatively small AC component with a phase orenvelope representative of rotor position. For example, if the voltagedifference between the lines 73a and 73b is 270 volts, the DC componentof the unenergized phase voltage would be approximately 135 volts, andthe average of the voltages of the two energized phase windings would beapproximately 135 volts. The relatively small AC component of theunenergized phase voltage might be one volt peak-to-peak.

In order to extract the small AC component of the unenergized phasevoltage, which contains the information regarding the angular positionof the rotor 20 with respect to the stator 26, the average phase voltagesignal generated by the divider 210 is provided to the inverting inputof the summing amplifier 200, where it is subtracted from theunenergized phase voltage, resulting in the AC component of theunenergized phase voltage which is representative of rotor position.

The positive portions of the pulses of waveforms T1-T6 shown in FIG. 8may be pulse-width modulated (not shown) by a carrier signal having amuch higher frequency than the waveforms T1-T6. As a result, the rotorposition signal generated by the summing amplifier 200 will have afrequency and phase the same as the PWM carrier frequency, but themagnitude, or envelope, of the signal will vary at a much lowerfrequency with a phase which is representative of rotor position.

To extract the lower frequency envelope from the rotor position signal,the output of the summing amplifier 200 is provided to a synchronousdemodulator circuit comprising a multiplier 212 and a low pass filter214. The multiplier 212 comprises an inverter 216 and a two-input switch218. A first input of the switch 218 is connected to receive the rotorposition signal from the amplifier 200, and a second output of theswitch 218 is connected to receive an inverted rotor position signalfrom the inverter 216. The switch 218 is switched at the frequency ofthe PWM carrier signal to alternately provide at its output theuninverted and inverted rotor position signal. This particularmultiplier circuit 212 is used in the case of a square-wave PWM carriersignal. Other types of multiplier circuits and synchronous demodulatorcircuits could also be used.

The demodulated rotor position signal is generated on a line 220 andprovided to a portion of the reluctance controller 166, shown in FIG.7B, which converts the rotor position signal into a clock signal andgenerates from that clock signal the six drive signals T1-T6 forcontrolling the transistor power switches T₁ -T₆ of the main inverter72. Referring to FIG. 7B, the rotor position signal on the line 220 isprovided to the negative input of a first comparator 230 and thepositive input of a second comparator 232. The first comparator 230determines when the magnitude of the rotor position signal is morenegative than a predetermined negative voltage, -V_(ref), and the secondcomparator 232 determines when the magnitude of the rotor positionsignal is greater than a predetermined positive voltage, +V_(ref).

The clock signal is generated from the output of the comparators 230,232 by a 1-of-2 data selector comprising a pair of AND gates 240, 242,an OR gate 244, and an inverter 246. A first binary data select signalis provided to the AND gate 242 via a line 252 and a second binary dataselect signal is provided to the AND gate 240 via the inverter 246connected to the line 252. The data select signals, which at all timesare complemented with respect to each other, are generated from theleast-significant bit (LSB) of a counter 250 so that the data selectsignals values switch each time the count of the counter increases byone.

The rotor position signal, the +V_(ref) and -V_(ref) signals, and theclock signal generated by the OR gate 244 are shown in FIG. 9. Therising edge of each pulse of the clock signal is triggered alternatelyby the magnitude of the rotor position signal reaching the predetermined+V_(ref) and -V_(ref) signals. The rising edge of each clock pulsetriggers the counter 250 to increase the count, causing the binary valueof the least-significant bit to change and the magnitudes of the dataselect signals to switch. As a result, the AND gate 240 or 242 thatgenerated the high binary value (when either the +V_(ref) or -V_(ref)magnitude was exceeded) is no longer selected, and the clock signalmagnitude falls to zero shortly after each rising edge.

The actual shape of the rotor position signal generated on the line 220would approximate the waveform produced by horizontally compressing thebold portions of the sinusoidal signal, which fall between the +V_(ref)and -V_(ref) thresholds, so that they were joined together. The actualshape would thus approximate a triangular waveform. The magnitude of theactual rotor position signal does not substantially exceed the +V_(ref)and -V_(ref) thresholds because, when the rotor position signal reacheseach threshold, the drive signals on the lines 85a-85f cause the rotor20 to be advanced 60 electrical degrees shortly thereafter. Themagnitudes of the +V_(ref) and -V_(ref) thresholds should be selected sothat the horizontal "spacing" between the bold portions of the rotorposition signal corresponds to 60 electrical degrees, as shown in FIG.9. The magnitudes of the +V_(ref) and -V_(ref) thresholds may beselected to be a predetermined percentage of the voltage across the DClink 54a-54b, the percentage being based upon generator parameters.

The counter 250 is a modulo-six counter having a three-bit binaryoutput. The output of the counter 250 is provided to a logic unit 260which generates the six transistor drive signals T1-T6 in accordancewith the counter output in accordance with the table shown in FIG. 10.

When the generator 10 is initially started, the counter 250 can beinitialized with one of six initial counts so that one set of drivesignals is generated on the lines 85a-85f. If the counter 250 does notincrement within a predetermined period of time, meaning that theinitial set of drive signals did not cause any torque to be produced onthe rotor 20, the counter 250 can be loaded with another one of the sixinitial counts until the correct count is loaded. The loading of theinitial counts could be performed by a digital signal processor coupledto the counter 250.

The 1-of-2 selector, the counter 250, the logic unit 260, and/or otherfeatures of the described embodiment could be implemented with a digitalsignal processor executing a computer program.

A portion of a first alternative embodiment is shown in FIG. 11. In thisembodiment, the phase windings 36a-36c are connected to three switches270a-270c, respectively, each of which has a single output connected tothe noninverting input of a summing amplifier 272. The switches270a-270c are switched such that the unenergized phase winding isconnected to the noninverting input of the summing amplifier 272. Eachof the phase windings 36a-36c is also connected to a respectivenoninverting input of a summing amplifier 274, which generates a signalrepresenting the sum of the phase winding voltages. The phase voltagesum is provided to a divider 276, which divides the phase voltage sum bythree to generate a signal representing the average phase voltage. Theaverage phase voltage is provided to the inverting input of theamplifier 272 whereby that voltage is subtracted from the voltage at theunenergized phase winding. The output of the summing amplifier 272 wouldbe provided to the multiplier circuit 212 of FIG. 7A.

A portion of a second alternative embodiment is shown in FIG. 12. Inthis embodiment, the phase windings 36a-36c are connected to threeswitches 280a-280c, respectively, which are switched so that theunenergized phase winding is connected to the noninverting input of asumming amplifier 282. The junction of the phase windings 36a-36c isconnected to the inverting input of the amplifier 282 so that thevoltage at the junction of the windings is subtracted from the voltageat the unenergized phase winding. The output of the summing amplifier282 would be provided to the multiplier circuit 112 of FIG. 7A.

As a further alternative, shown in the left-hand portion of FIG. 12,instead of connecting the inverting input of the amplifier 282 to thejunction of the phase windings 36a-36c, the inverting input could becoupled, via dotted line 290, to the junction of a pair of identicalresistors 292, 294 provided across the DC link conductors 73a-b so thatthe inverting input of amplifier 282 would receive a signal representingone-half the voltage across the DC link conductors 73a-73b.

A portion of a further alternative embodiment is illustrated in FIG. 13.In the embodiment of FIG. 13, rotor position is detected based oninductance sensing instead of differential reluctance sensing. Referringto FIG. 13, the voltages on all three phase windings 36a-36c areprovided to a summing amplifier 300, which generates a signalrepresenting the voltage sum. The voltage sum signal is provided to adivider 302, which divides the voltage sum signal by three to generatean average phase voltage signal on a line 303. The voltage at thejunction of the wye-connected windings 36a-36c is provided via a line304 to the noninverting input of a summing amplifier 306. The summingamplifier 306 generates a rotor position signal by subtracting theaverage phase voltage signal on the line 303 from the junction voltageon the line 304. In the embodiment of FIG. 13, the average phase voltagesignal on the line 303 could be generated in other ways. For example, itcould be generated by dividing the sum of the voltages on the energizedphase windings by two, or it could be generated by dividing the DC linkvoltage by two.

Back-EMF controller 160

The structure and operation of the back-EMF controller 160 is describedbelow in connection with FIGS. 14-19. FIG. 14 is a block diagram of theback-EMF controller 160 shown schematically in FIG. 5 along with themain inverter 72 and the main generator portion armature windings36a-36c. The back-EMF controller 160 includes a rotor position detector340 and a starting system control 341 for operating the generator 10 ina starting mode to convert electrical power into motive power. The rotorposition detector 340 includes a back EMF state observer 342 and anangle reconstruction unit 344. The back EMF state observer 342 isresponsive to the phase voltage signals V_(a) and V_(b) on the lines87a-b and the phase current signals I_(a) and I_(b) on the lines 89a-band produces a back EMF estimate signal E_(ab) indicative of the backEMF voltage. The angle reconstruction unit 344 responds to the back EMFestimate signal E_(ab) to develop signals representing the position andspeed of the motive power shaft 18 which are delivered to a speedcontroller 346.

The speed controller 346 develops a torque command signal on a line 348representing the commanded torque to be produced by the generator 10 anda further signal on a line 350 representing the mechanical position ofthe motive power shaft 18. The signal on the line 350 is summed with aphase advance signal by a summer 354. The phase advance signal isdeveloped by a function generator 356 and is dependent upon the speed ofthe motive power shaft 18 as detected by the angle reconstruction unit344. The function generator 356 provides increasing phase advance asspeed increases in a high speed range. The summer 354 develops anelectrical angle command signal on a line 358 which is supplied to firstand second functional blocks 360, 362 which generate a cosine waveformsignal and a sine waveform signal, respectively, each of which has thesame frequency as the electrical angle command signal on the line 358.

Second and third multipliers 364, 366 are coupled to the functionalblocks 360, 362, respectively, and multiply the outputs thereof with thetorque command signal on the line 348. The output signals generated bythe multipliers 364, 366 are provided to a 2-to-3 phase converter 372via a pair of lines 368, 370. The phase converter 372 converts thosesignals into three-phase sinusoidal current signals which are in turnsupplied to three summers 374, 376, and 378 via lines 375, 377, and 379,respectively.

Each of the summers 374, 376, and 378 sums one of the three-phasecurrent signals produced by the phase converter 372 with a signalrepresenting the magnitude of a phase current of the AC power to obtainan error signal. In the case of the summers 376 and 378, the phasecurrent magnitudes are detected by current sensors 88a-88b while thephase current magnitude for the summer 374 is obtained by a summer 384which adds (in a negative sense) the magnitudes developed by the currentsensors 88a-88b. The error signals are processed by gain andcompensation units 386, 388, and 390, preferably of theproportional-integral type, and are provided to a waveform generator 392which generates the commutation signals and provides them to the maininverter 72.

The back EMF state observer 342 estimates the back EMF voltage producedby the generator 10 while it is operating in the starting mode. Duringthe starting mode, currents flow in the armature phase windings 36a-36caccording to the following current state equation: Where: V_(ab) is theline-to-line voltage; ##EQU1## E_(ab) is the line-to-line back EMFvoltage across the lines 48a and 48b;

I_(ab) is the difference between the current signals I_(a) and I_(b)produced by the current sensors 88a, 88b, respectively;

L is the armature winding inductance; and

R is the armature winding resistance.

Although the above formula can be implemented by differentiating thecurrents flowing in the main generator portion armature windings36a-36c, known differentiation processes are vulnerable to noise. Toavoid differentiation, the back EMF state observer 342 is configured asshown in FIG. 15.

Referring to FIG. 15, the voltages V_(a) and V_(b) appearing at thelines 48a and 48b, respectively, are delivered to a summer 394, anoutput of which is passed through a low pass filter 396 to produce theline-to-line voltage signal V_(ab). The current signal I_(b) issubtracted from the current signal I_(a) by a summer 400 to produce thecurrent magnitude signal I_(ab). The current magnitude signal I_(ab) isdelivered to an inverting input of a summer 402 while a current estimatesignal I_(ab), produced as described below, is delivered to anon-inverting input of the summer 402. The output of the summer 402 isdelivered to a conditioner 404, which preferably comprises aproportional-integral type compensator, but which could also comprise anonlinear controller. The conditioner 404 produces the back EMF estimatesignal E_(ab) and delivers such signal to an inverting input of a summer406 and to the angle reconstruction unit 344 of FIG. 14. The currentestimate signal I_(ab) is delivered to a conditioner 408 preferablycomprising a gain unit 10 with a gain value of R and having an outputcoupled to an inverting input of the summer 406. The summer 406subtracts the current estimate signal I_(ab) as conditioned byconditioner 408 and the back EMF estimate signal E_(ab) from theline-to-line voltage signal V_(ab) developed by the low pass filter 396.An output signal developed by the summer 406 is conditioned by aconditioner 410, preferably comprising a gain unit having a gain valueof 1/L, and integrated by an integrator 412 to produce the terminalcurrent estimate signal I_(ab).

Although the back EMF state observer 342 is shown implemented by analogcomponents, it should be noted that the back EMF state observer 342could, instead, be implemented by a microprocessor suitably programmedto perform the functions described above.

Referring now to FIG. 16, a first embodiment of the angle reconstructionunit 344 is shown in greater detail. This embodiment comprises aphase-locked loop which responds to the back EMF estimate signal E_(ab)to develop the indication of the angular position of the motive powershaft 18 at an output 414. Zero crossing detectors 416 and 418 detectthe zero crossings of the back EMF estimate signal E_(ab) and theangular position indication developed at the output 414 to producesquare wave signals which are delivered to a phase detector 420. Thephase detector 420 compares the outputs of zero crossing detectors 416and 418 to produce an error signal indicative of the phase differencebetween the back EMF estimate signal E_(ab) and the angular positionindication. The integrator 422 integrates the error signal and theintegrated error signal is passed via a line 423 to a voltage-to-angleconverter 424, which preferably comprises a voltage-controlledoscillator. The voltage-to-angle converter 424 produces a ramp signalhaving a magnitude that is indicative of the angular position of themotive power shaft 18.

The signal generated on the line 423, which is generated by theintegration of the phase errors detected by the phase detector 420, hasa magnitude that increases with frequency. Thus, the signal on the line423 is representative of the estimated speed of the motive power shaft18.

FIG. 16A illustrates a first embodiment of the voltage-to-angleconverter 424 shown schematically in FIG. 16. The converter 424 includesa voltage-to-frequency (V/F) converter 425 connected to receive thespeed signal output by the integrator 422 (FIG. 16). Based on themagnitude of the speed signal, the V/F converter 425 generates an outputsignal of a particular frequency that is provided to the count input ofa counter 426. The count signal generated by the counter 426 is providedto a digital-to-analog (D/A) converter 427, which generates a rampsignal having a magnitude related to rotor position. In operation, asthe magnitude of the speed signal on the line 423 increases, thefrequency of the signal generated by the V/F converter 425 increases,thus increasing the rate at which the counter 425 counts and increasingthe frequency of the ramp signal generated by the D/A converter 427.Preferably, the counter 426 automatically resets, such as a modulocounter, when its count reaches a predetermined value, thus causing thedownward portions of the ramp signal to occur automatically.

FIG. 16B illustrates a second embodiment of the voltage-to-angleconverter 424 shown schematically in FIG. 16. The converter 424 includesan integrator 428 connected to receive the speed signal output by theintegrator 422 (FIG. 16). The integrator 428 generates a ramp signalhaving a magnitude related to rotor position. The integrator 428 isperiodically reset by the output of a level comparator 429 when theintegrator output reaches a predetermined level, which causes thedownward sloping portions of the ramp signal.

Referring now to FIG. 17, an alternative embodiment of the anglereconstruction unit 344 is shown in greater detail. The back EMFestimate signal E_(ab) is supplied to an interface amplifier 430 and azero crossing detector 432 which produces a narrow pulse each time theE_(ab) signal rises upwardly through the zero axis, which occurs every360 electrical degrees. The output of the zero crossing detector 432 isprovided to a delay circuit 434 which in turn provides a reset signal toa counter 436. The counter 436 accumulates clock pulses produced by aclock 438 and is reset every 360° of the back EMF estimate signalE_(ab). The output of the counter 436 represents the time that elapsesbetween consecutive pulses produced by the zero crossing detector 432.The falling edge of each pulse comprises a write command to a latch 440which latches the output of the counter 436. The output of the latch 440is inverted, i.e., the reciprocal thereof is calculated, by a circuit442, which generates a signal having a magnitude that is representativeof the speed of the motive power shaft 18, and thus of the rotor 20.

The speed signal generated by the circuit 442 is supplied to anintegrator 444, which integrates the speed signal to generate a signalhaving a magnitude that is representative of the angular position of themotive power shaft 18, and thus of the rotor 20. The integrator 444 isreset every 360° of rotation of the rotor 20 via a reset signalgenerated by a delay circuit 446 connected to receive the 360°-spacedpulses generated by the zero crossing detector 432 so that a ramp-shapedposition signal is generated.

FIG. 18 illustrates in greater detail the speed controller 346 shownschematically in FIG. 14. In response to the rotor speed and rotorposition signals generated by the angle reconstruction unit 344, thespeed controller 346 generates an angle command and a torque command.

Referring to FIG. 18, a speed command signal is provided via a line 450to the non-inverting input of a summer 452. The speed command maycomprise a step voltage from a first voltage to a second, higher voltageor may comprise any other type of waveform as desired. The output of thesummer 452 is coupled to a function generator 454 which develops anacceleration command signal which is, in turn, integrated by anintegrator 456 to produce a speed reference signal. The speed referencesignal is fed back to an inverting input of the summer 452, and hencethe elements 452, 454, and 456 comprise a closed-loop circuit.

The speed reference signal is integrated by an integrator 458 to developa position reference signal which is provided to the non-inverting inputof a summer 462. The rotor position signal from the angle reconstructionunit 344 of FIG. 14 is also provided to an inverting input of the summer462. The summer 462 produces a position error signal indicative of thedifference between the derived position reference signal and the actualrotor position developed by the angle reconstruction unit 344. Theposition error signal is provided to a gain unit 476 and is supplied toa first input of a summer 478.

A speed error signal is developed by a summer 480 which subtracts therotor speed signal developed by the angle reconstruction unit 344 ofFIG. 14 from the speed reference signal developed by the integrator 456.The speed error signal is processed by a gain unit 482 and is summedwith the output of the gain unit 476 by the summer 478. A limiter 484provides limiting for the output of the summer 478 and develops acurrent limited torque command signal on the line 348 of FIG. 14.

FIG. 19 illustrates in greater detail the phase converter 372 shownschematically in FIG. 14. The converter 372 includes three operationalamplifiers 490, 492, 494 and associated biasing circuitry connected tothe lines 368, 370 which generate outputs on the lines 375, 377, 379.

Numerous modifications and alternative embodiments of the invention willbe apparent to those skilled in the art in view of the foregoingdescription. Accordingly, this description is to be construed asillustrative only and is for the purpose of teaching those skilled inthe art the best mode of carrying out the invention. The details of thestructure may be varied substantially without departing from the spiritof the invention, and the exclusive use of all modifications which comewithin the scope of the appended claims is reserved.

We claim:
 1. A control for a brushless generator having a main generatorportion including a field winding disposed on a rotor and which receivesfield current and an armature winding disposed in a stator wherein therotor is movable with respect to the stator and a permanent magnetgenerator (PMG) having an armature winding coupled to a rectifier whichis coupled to a voltage regulator wherein the generator is operable in agenerating mode to convert motive power into electrical power and in astarting mode to convert electrical power provided to the main generatorportion armature winding into motive power, comprising:an exciter havingan exciter field winding disposed in the stator and an armature windingdisposed on the rotor and coupled to the main generator portion filedwinding; first and second power converters each having an input and anoutput; means operable in the starting mode for coupling a source ofelectrical power to the inputs of the first and second power converters,the output of the first power converter to the main generator portionarmature winding and the output of the second power converter to theexciter field winding and operable in the generating mode fordisconnecting the source of electrical power from the first and secondpower converters and connecting the voltage regulator to the exciterfield winding and the input of one of the power converters to the maingenerator portion armature winding; and means coupled to the first andsecond power converters and operable in the starting mode forcontrolling the power converters such that the first and second powerconverters provide first and second AC waveforms at first and secondfrequencies to the main generator portion armature winding and to theexciter field winding, respectively, at the beginning of a star sequenceand such that the second AC waveform changes to a third frequency lessthat the second frequency after the beginning of the start sequence andoperable in the generating mode for controlling the power convertercoupled to the main generator portion armature winding such that ACpower is produced at the output of such power converter.
 2. The controlof claim 1, wherein the controlling means comprises means for causingthe frequency of the second AC waveform to continuously decrease duringthe start sequence until a particular rotor speed is reached.
 3. Thecontrol of claim 2, wherein the causing means includes means forapplying DC power to the exciter field winding after the particularrotor speed is reached.
 4. The control of claim 3, wherein the applyingmeans decreases a parameter of the DC power after a further particularrotor speed is reached.
 5. The control of claim 4, wherein the parameterof DC power comprises DC voltage magnitude.
 6. The control of claim 1,wherein the controlling means comprises means for causing the frequencyof the second AC waveform to uniformly and continuously decrease duringthe start sequence.
 7. The control of claim 1, wherein the exciter fieldwinding includes a mid-tap and first and second end taps and wherein thecoupling means includes means for connecting the mid-tap and one of theend taps to the second power converter during operation in the startingmode and for connecting the end taps to the voltage regulator in thegenerating mode.
 8. The control of claim 1, wherein the first and secondpower converters comprise first and second inverters, respectively, andwherein an AC/DC converter is coupled between the source of electricalpower and the first and second inverters during the start sequence. 9.The control of claim 1, wherein the controlling means comprises meansfor causing the frequency of the first AC waveform to uniformly andcontinuously increase during the start sequence.
 10. The control ofclaim 1, wherein the controlling means includes means for causing thefrequency of the first AC waveform to continuously increase during thestart sequence.
 11. The control of claim 10, wherein the causing meansincludes means for detecting rotor position and means for commutatingthe main generator portion armature winding based upon the detectedrotor position.
 12. The control of claim 11, wherein the detecting meanscomprises a sensorless rotor position detector.
 13. A control for abrushless generator having a main generator portion including a fieldwinding disposed on a rotor and which receives field current and a setof armature windings disposed in a stator wherein the rotor is movablewith respect to the stator and a permanent magnet generator (PMG) havinga set of armature windings coupled to a rectifier which is coupled to avoltage regulator wherein the generator is operable in a generating modeto convert motive power into electrical power and in a starting mode toconvert electrical power provided to the set of main generator portionarmature windings into motive power, comprising:an exciter having anexciter filed winding disposed in the stator and a set of armaturewindings disposed on the rotor and coupled to the main generator portionfield winding by a set of rotating rectifiers; first and secondinverters and a rectifier bridge each having an input and an output;contactors operable in the starting mode for coupling a source of ACpower to the input of the rectifier bridge, the output of the rectifierbridge to the input of the first and second power inverters, the outputof the first inverter to the set of main generator portion armaturewindings and the output of the second inverter to the exciter fieldwinding and operable in the generating mode for disconnecting the sourceof AC power from the first and second inverters and connecting thevoltage regulator to the exciter field winding, the input of therectifier bridge to the set of main generator portion armature windingsand the input of one of the inverters to the output of the rectifierbridge; and means coupled to the first and second inverters and operablein the starting mode for controlling the inverters such that the firstand second inverters provide first and second AC waveforms at first andsecond frequencies to the set of main generator portion armaturewindings and to the exciter field winding, respectively, at thebeginning of a start sequence and such that the frequency of the secondAC waveform continuously decreases during the start sequence until aparticular rotor speed is reached after the beginning of the startsequence and operable in the generating mode for controlling theinverter coupled to the main generator portion armature winding suchthat AC power is produced at the output of such inverter.
 14. Thecontrol of claim 13, wherein the controlling means includes means forapplying DC power to the exciter field winding after the particularrotor speed is reached.
 15. The control of claim 14, wherein theapplying means decreases a parameter of the DC power after a furtherparticular rotor speed is reached.
 16. The control of claim 15, whereinthe parameter of DC power comprises DC voltage magnitude.
 17. Thecontrol of claim 13, wherein the exciter field winding includes amid-tap and first and second end taps and wherein the contactors connectthe mid-tap and one of the end taps to the second inverter duringoperation in the starting mode and connect the end taps to the voltageregulator in the generating modes.
 18. The control of claim 13, whereinthe controlling means includes means for causing the frequency of thefirst AC waveform to continuously increase during the start sequence.19. The control of claim 18, wherein the causing means includes meansfor detecting rotor position and means for commutating the set of maingenerator portion armature windings based upon the detected rotorposition.
 20. The control of claim 13, wherein the controlling meanscomprises means for causing the frequency of the first AC waveform touniformly and continuously increase during the start sequence.
 21. Thecontrol of claim 19, wherein the detecting means comprises a sensorlessrotor position detector.